Some background

The tags act to prevent lots of pages being indexed, instead of just one main page.

One of the common uses of canonicals is in ecommerce on category pages, where many versions of the URL exist, but the retailer only wants to rank for retailer.com/shirts, for example.

Let's look at another example in ecommerce - competing product pages - with the help of Pi Datametrics, who investigated this issue at Waterstones, presenting findings at Brighton SEO last month.

Waterstones - hardback versus softback

Pi Datametrics took a look at Waterstones' search performance for certain novel titles.

The chart below shows a Waterstones' product page (The Bone Clocks in Hardback) and its highest daily position for a search term ('The Bone Clocks').

As you can see, performance is patchy, with regular drops from the top 10 to way down the SERPs (100+).

So, what is causing this flux?

Looking at a second chart showing the rankings of two Waterstones' pages (this time both the hardback and the paperback), a pattern emerges.

When the ranking of the hardback URL (in pink) drops off, the paperback URL (in yellow) can sometimes be seen to take its place.

So, how to stop this ranking switching?

The canonical book

Of course, popular books have many versions - hardback, paperback, special editions, audio, e-book and so on, often with reviews sitting on different web pages, too.

Perhaps what Waterstones needs to do to ensure consistently high ranking for this term, 'The Bone Clocks', is to choose a 'canonical book'.

Pi Datametrics illustrates this in the following diagram, with canonical tags on newer editions pointing back to the original book's URL (and eventually the paperback, which will be the default canonical).

The canonical book will also reciprocally link with the other formats (that need to rank separately), such as audio and e-book.

Did it work?

Waterstones did indeed implement this 'canonical book' approach.

And the chart below speaks for itself. Lovely, consistent top 10 results for Waterstones when searching for 'The Bone Clocks' (from November onwards).

The paperback URL (yellow) is always the product page that ranks highest, and in doing so it leaves fewer gaps that the competition can exploit.

This is a really clear and simple demonstration of the need for canonical tags for certain products that have multiple versions, which then affect Google ranking performance.

Thanks to Jon Earnshaw who discussed this case study at Brighton SEO for Pi Datametrics.

As part of the same presentation, Jon looked at how similar product pages from competitors can scupper search performance and how you can combat this.

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